The Supreme Court's decision this year in Citizens United, which lifted campaign spending restrictions for companies and interest groups, has indirectly thrust the Internal Revenue Service into the more prominent role of overseeing those expenditures.

The ruling largely tied the hands of the Federal Election Commission to force companies or groups that are funding political or advocacy advertisements to disclose their donors. And although Democrats in Congress had pushed legislation that would have mandated more disclosure, the measure failed in the Senate this summer.

Long-standing IRS regulations require some groups to reveal their donors, and that is why the agency suddenly finds itself with what some might see as a more crucial watchdog role, stepping in to monitor disclosure in the absence of the FEC. But the IRS rules also have long-standing loopholes and, with limited resources and enforcement tools, the nation's tax collector is not set up to be a campaign regulator.

"The chances of the IRS being able to catch a violation of the tax law around campaigns is virtually nil," said Marcus S. Owens, a lawyer with Caplin & Drysdale who directed the agency's tax-exempt organizations division for 10 years. "Certainly if it happens, it's going to be well after the election has already ended."

Since the court's ruling, some prominent Democrats and others have argued that, with much more freedom to spend on campaign ads, corporations, unions and advocacy organizations ought to be forced to disclose who is paying for the ads and, in the case of groups, where the money is coming from.

The IRS, which declined to comment, requires groups whose "primary purpose" is political activity to name their donors. But that requirement is open to wide interpretation.

The conservative group Americans for Job Security, for example, founded in 1997 as a business association, spends the vast majority of its budget on television and radio ads before elections.

At the end of July, it disclosed spending $585,000 on an ad attacking Colorado Republican Senate candidate Jane Norton, who recently lost her primary contest against Ken Buck, a county district attorney with "tea party" backing. The ad, running just ahead of the primary, said, "Norton passed the largest tax hike in Colorado history" and "Jane Norton's real record has cost us plenty."

The ad is similar to most run by Americans for Job Security -- criticizing politicians before an election. That fits under the definition of lobbying and not election activity, according to the group, which has declared itself in the advocacy category and therefore is not required to disclose its donors to the IRS.

"What we do is grass-roots lobbying and issue advocacy," Executive Director Steve DeMaura said. "Elections are when the American people are most engaged in educating themselves about public policy issues."

Ideological groups taking the form of nonprofit "social welfare" organizations -- which also are not required to disclose -- include FreedomWorks, the anti-tax group fueling the tea party movement, and American Crossroads GPS, a new organization associated with Karl Rove, an adviser in the George W. Bush administration.

Since the court's ruling, some groups have opted to disclose donor information to the FEC. But others, including some of the largest, have protected donors' identities by registering with the IRS as nonprofits whose primary purpose is advocacy rather than politics.

The IRS has limited ability to enforce its rules. It cannot fine groups for violating their tax status, for example. Its only option is to charge taxes on specific types of expenditures or take the much bigger step of revoking a group's tax-exempt status -- a punishment often considered incommensurate with most infractions.

"The only effective tool the IRS has to use against a nonprofit is the death penalty," said Brett Kappel, a lawyer with the firm Arent Fox.

One of the few times the IRS has denied tax-exempt status was in 1999 for the Christian Coalition, which operated as a social welfare organization while distributing "voter guides" to churches favoring certain candidates. The battle became highly politicized, with legal challenges that stretched out for years.

Also hampering the agency's ability to enforce is the fact that it operates on what Owens calls "tax time." Any investigation comes after an organization has filed a tax return. Nonprofits are often granted two three-month extensions before they file, meaning most returns showing campaign spending this fall will come about a year after the election.

When returns detailing election-related spending are filed, they fit into a queue with 1.9 million other tax-exempt organizations, including charities, foundations and clubs. The agency's 200 revenue agents focused on tax-exempt filings have traditionally focused on making sure that the groups don't abuse tax-deductible gifts or tax-exempt status for private gain.

Watchdog groups complain that when they have flagged alleged violations, their notices seem to go nowhere.

"The IRS doesn't respond," said Fred Wertheimer, president of Democracy 21, which advocates for increased regulation of money in politics. "Sometimes they say 'we have received your letter,' but you never hear from them about the status."